1. Field of the Invention
The invention generally relates to communications in the Circuit Switched Fall-Back (CSFB) architecture, and more particularly, to circuit service tunneling methods and user equipments thereof which conform to the CSFB architecture.
2. Description of the Related Art
With growing demand for mobile communications, the Global System for Mobile communications (GSM) supporting only circuit-switched (CS) domain services no longer meets user requirements. The mobile communications working groups and standard groups accordingly have developed the so-called third generation mobile communications technologies, such as Wideband Code Division Multiple Access (W-CDMA), Code Division Multiple Access-2000 (CDMA-2000), Time Division-Synchronous Code Division Multiple Access (TDSCDMA), etc. Take the Universal Mobile Telecommunications System (UMTS) using the W-CDMA technology for example. The 3rd Generation Partnership Project (3GPP) has further proposed the Long Term Evolution (LTE) system, also called the fourth generation mobile communications system, to be implemented to meet future demand for large wireless data transmission bandwidths. The LTE system aims to provide an all-IP architecture, in which only packet-switched (PS) domain instead of CS domain is used to carry all mobile communications services. Also, voice services are provided by Voice over Internet Protocol (VoIP) in the fourth generation mobile communications system, instead of the CS domain of the third generation mobile communications system.
However, during transition from the third generation mobile communications system to the fourth generation mobile communications system, some operators have still chosen to provide voice services via the CS domain of the third generation mobile communications system, due to the situation that the coverage rate and capacity of the CS domain of the third generation mobile communications system are already sufficient. A problem is raised for that the architecture of the third generation mobile communications system comprises both CS domain and PS domain while the fourth generation mobile communications system comprises only PS domain. Accordingly, the interconnection between the third generation mobile communications system and the fourth generation mobile communications system, especially the CS domain part, must be defined and specified, so that systems of different standards can smoothly interwork to provide voice services to users. One technique used is the Circuit Switched FallBack (CSFB) architecture, which is defined in the 3GPP TS 23.272 specification.
It is noted that, in the interconnection between the LTE system and the CDMA-2000 system, the 1x Link Access Control (1x LAC) layer of the CDMA-2000 protocol stack processes logical channels with a specific mapping relationship between the channels. Consequently, the 1x LAC layer in the CDMA-2000 system does not provide logical channel indication information and the CDMA-2000 system does not provide information of the logical channel used for transmitting signaling messages of the 1x LAC layer. In the current CSFB architecture, upon receiving a signaling message of the 1x LAC layer from the CDMA-2000 system, the 1x LAC layer in the LTE system can not obtain correct logical channel information to process the signaling messages of the 1x LAC layer. In addition, the 1x 2nd layers in the current CSFB architecture do not have an acknowledgement mechanism to ensure the delivery of some signaling messages of the 1x LAC layer, e.g. the registration messages. Thus, a method is needed to correctly interconnect the LTE system and the CDMA-2000 system operating in the CSFB architecture.